Something I have always done was write down the colors of paints I used when painting armies. I have found it helpful in case I had to touch up a model or add a new model to an existing army, I could better match the model to the army. I have done this in the past by using 3x5 cards just kept in a box on a shelf.
What I do is just write down some notes while I am painting the models on the 3X5 card. Nothing earth shattering with this. I might change some colors after painting a few models and I would just edit my notes. Once I decided on my colors I would just rewrite my notes to a final version and call it good.
Today I went out and purchased a water color note book so I can catalog my paints better and even some of my paint recipes and techniques. This is important to me because I use so many different brands of paint and have so many different shades of the same colors. This month I have really been working with Red and this is my collection of Red paints and Inks.
My goal is first and for most go through my paints and make sure they are still good and catalog them in my new journal. I should be able to practice some on my new techniques on the water color paper it is designed for painting and I have not had any issues with my other waster color note pad. Then I will keep all the different brands together in one storage box so when I am looking for Red I go to one place and have all my paints. I can also just flip to that page on my journal and I can see the differ colors better and read my notes on how I have used them. I feel this is going to be very helpful for me as I move forward developing my painting skills.